Light olefins, defined herein as ethylene and propylene, serve as feeds for the production of numerous chemicals. Olefins traditionally have been produced by petroleum cracking, for example, by fluidized catalytic cracking (FCC). Because of the limited supply and/or the high cost of petroleum sources, the cost of producing olefins from petroleum sources has increased steadily.
In addition to cracking petroleum products, the petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into light olefins. The preferred conversion process is generally referred to as an oxygenate to olefin (OTO) reaction process. Specifically, in an OTO reaction process, an oxygenate contacts a molecular sieve catalyst composition under conditions effective to convert at least a portion of the oxygenate to light olefins. When methanol is the oxygenate, the process is generally referred to as a methanol to olefin (MTO) reaction process. Methanol is a particularly preferred oxygenate for the synthesis of ethylene and/or propylene
In an OTO conversion process carbonaceous material (coke) is deposited on the molecular sieve catalysts used to catalyze the conversion process. Excessive accumulation of these carbonaceous deposits will interfere with the catalyst's ability to promote the reaction. In order to avoid unwanted build-up of coke on molecular sieve catalysts, the OTO and MTO processes incorporate a second step comprising catalyst regeneration. During regeneration, the coke is at least partially removed from the catalyst by combustion with oxygen, which restores the catalytic activity of the catalyst and forms a regenerated catalyst. The regenerated catalyst then may be reused to catalyze the OTO conversion process.
In a conventional regeneration system, coked catalyst is directed from a reactor to a catalyst regenerator. In the catalyst regenerator, a regeneration medium, usually oxygen, enters the regenerator, and coke is removed from the coked catalyst by combustion with the regeneration medium to form regenerated catalyst and gaseous byproducts. The bulk of the regenerated catalyst from the regenerator is returned to the reactor. The gaseous byproducts are forced out an exhaust outlet oriented in the upper section of the catalyst regenerator.
The combustion of the carbonaceous deposits from molecular sieve catalyst compositions during catalyst regeneration is an exothermic process. The exothermic nature of catalyst regeneration presents a unique problem in OTO regeneration systems because OTO reaction systems typically operate with catalyst that is significantly more coked than catalyst implemented in FCC reaction systems because operating at higher coke levels provides an increased selectivity to light olefins. As a result, the amount of heat liberated from the OTO molecular sieve catalyst compositions during catalyst regeneration can be greater than the amount of heat liberated from the regeneration of catalysts in other catalytic processes such as FCC reaction processes.
In some designs, the coked catalyst is transported in a fluidized manner from the reactor to the catalyst regenerator. This transportation regime can comprise contacting the coked catalyst with fluidizing medium, such as air, in a conduit under conditions effective to transport or lift the catalyst in a fluidized manner from the reactor to the catalyst regenerator. When the coked catalyst contacts an oxygen-containing fluidizing medium such as air, however, the regeneration process may begin in the conduit itself rather than in the catalyst regenerator.
It has now been discovered that due to the relatively high coke content on OTO molecular sieve catalyst compositions, a significant amount of heat may be liberated within the conduit as the coked catalyst is transported through the conduit. In OTO reaction systems, this heat potentially possibly could exceed the material tolerances of the materials used to form the conduit. The heat also can damage the catalyst particles themselves. As a result, improved processes are sought for transporting coked catalyst to a catalyst regenerator in OTO reaction systems.